Modern radio receiver designs often require amplifiers for increasing the power level of input radio frequency (RF) signals. An amplifier for use in this application would ideally have a low noise figure, low power consumption, high gain and a high degree of linearity. Moreover, such amplifier would ideally further have a broad bandwidth, automatic gain control (AGC) capability, low cost, small size and a high degree of reverse isolation.
There are many prior art amplifier circuits some of which might meet some of the foregoing criteria. One prior art amplifier includes a single field effect transistor (FET) connected in a common source configuration which tends to undesirably provide narrow band operation, but has desirably low noise figures for some applications. Other prior art amplifier designs utilize a bipolar transistor and voltage controlled attenuators which provide wide band operation and desirable noise figures, but these circuits are generally expensive and consume an undesirable amount of power for some applications. Still other prior art preamplifiers do not have AGC capability.
Moreover, some prior art amplifier designs undesirably couple signals generated in circuits connected to the output terminals thereof to circuits coupled to the input terminals thereof. Such circuits do not have a sufficiently high degree of "reverse isolation" and they are not suitable for use as receiver preamplifiers because they allow local oscillator signals to be radiated by the receiver antenna, for instance.